Various processes are presently used commerically for shaping articles from thermoplastic materials such as polypropylene, polyethylene, and the like. It is common practice to shape a thermoplastic material by heating the material slightly below its melting point and extruding, sheet forming, press-brake forming or forging the material to a required shape. If the material is heated to a temperature exceeding its melting point, the resulting relatively liquid state of the material permits to shape it into an article of desired configuration and dimensions in an appropriate mold by injection molding.
Attempts have been made to form articles from thermoplastic materials at room temperature, but the results achieved have not been very satisfactory. This is due to the fact that, although most thermoplastic materials have relatively good ductility at room temperature, excessive bending and deep drawing create important local strains, which cause breakage and tears and undesired local thinning, and the excessive elastic and plastic strain recovery of the materials create many problems in designing adequate forming tools and in insuring commercially acceptable repeatability in shape accuracy and dimensions in mass production operations. In addition, complex shapes are hard to obtain by room temperature forming.
Articles of complex shape can however be easily obtained with good dimensional stability by injection molding. Unfortunately, injection molding presents many inconveniences and disadvantages, such as high tooling costs and low production rates. The low production rates of injection molding are due to the relatively slow mold opening rates, the time required for allowing the material to flow into the mold and throughout the mold cavity, the time required for permitting the article to solidify and to cool to a temperature low enough to enable it to maintain its shape and be cool enough to handle upon opening of the mold and ejection from the mold, and to the time required for opening the mold and removing the article therefrom.
Solid state molding processes, on the contrary, allow relatively high production rates, and permit utilization of techniques having some similarity with the techniques involved in sheet metal forming. However, conventional techniques of solid state forming of thermoplastic materials are limited in their applications as the amount of material flow is relatively small and generally less than can be achieved in the shaping of metals and these techniques are unable to readily mold parts having varying thicknesses, bosses or ribs.
The process of solid state forming of thermoplastic materials according to the present invention presents over prior art solid state molding techniques the advantages of high material flow and high production rates, the ability to form parts with varying thicknesses, bosses and ribs, and presents over injection molding techniques the advantage of relatively low tooling costs and high production rates. The present invention permits to form a solid blank of thermoplastic material to any appropriate shape with a mold closure time of less than 5 seconds. The present invention, in addition, permits to apply to the shaping of thermoplastic materials techniques having some analogy with the techniques used in the forming of metals, with the result that machines and apparatus currently used in metal forming may be readily adapted to the process of the invention, and that high production rates, complicated forms and high material flow are easily achieved by the invention.